This is a tough one! Often times, as I'm sure you're finding, you are limited to a Market Comparative Analysis - as you are attempting to do now. In situations such as this, if it's a multi-family, an appraiser might use Gross Rent Multiplier and/or CAP rate and compare to other large multi-family properties in the town/city/area code. Sometimes an appraiser might also use the cost comparative method (cost to replace the asset). In this case, you might want to try and leverage a R/E agent in the area and get their take.
Personally, it's risky, since you can't be as sure of the ARV. If you have your heart set on this building though, I'd use the Gross Rent Multiplyer (how many months it would take to pay the property off with the "gross rent") and the CAP rate - then connect through facebook (join the REIA group on FB) and post a quick question to the audience to see if those numbers look appropriate for the given area that you are investing in.
Best of luck! Feel free to reach out with any questions.